Single-layer or multilayered ceramic substrate materials form important starting materials for microelectronic components, in particular for telecommunication components. A layer represents a ply or a film of the substrate material having a large size/dimension in two spatial directions, and a comparatively small size/dimension in the third spatial direction oriented perpendicularly to the first two spatial directions. A sintered layer of an LTCC (low temperature cofired ceramics) substrate typically has a thickness of 140 μm.
Glass-ceramic materials are used to manufacture both materials having low dielectric constants, and also materials having moderately high dielectric constants (∈r) and/or relative permittivity (also referred to as dielectric index or permittivity index). A glass-ceramic material is a material which initially represents a glass composition, and which sinters into a dense compound upon subsequent tempering at temperatures up to 1000° C. before the crystallization, so that a partially or completely crystalline material arises.
An important and cost-effective technology for the production of microelectronic substrates having high occupation density is the so-called “low temperature cofired ceramics” technology, referred to hereafter as LTCC technology. LTCC technology is a technology for producing multilayered circuits based on sintered ceramic carriers. Printed conductors, capacitors, resistors, and coils may be generated on the substrates. These elements are applied to the unfired layer by screenprinting or photochemical processes. The unfired ceramic films are structured individually and subsequently stacked and laminated. A defined sintering profile having a peak temperature of approximately 850° C. to 900° C. is then applied to solidify the structure.
Single-layer or multilayered substrates for electronic configurations are frequently provided with materials which have a low dielectric constant to achieve high-speed transmission of electronic signals through the substrate at high frequencies. In the event of a low dielectric constant or permittivity, a large part of the high frequency signal (of an antenna, for example) is emitted, and little energy is coupled into the material and thus wastefully consumed.
A ceramic substrate material and a method for its production is known from DE 10 2007 020 888.
The porosification procedure, i.e., the etching step, in which the cavity structure is generated in the matrix of the known ceramic substrate material, allows the implementation of a dielectric constant which is between that of air (approximately 1) and the remaining glass ceramic or crystalline ceramic. In this case, dielectric constants ∈r resulting in the porosified areas—which are composed of the dielectric constant of the material of the etched layer remaining after the etching procedure, and the dielectric constant of the cavity structure—may be achieved with values of up to 2. The cavity structure is preferably formed by a pore and/or tube structure.
In the etched areas, the resulting dielectric constant ∈r is between approximately 10 and 1, especially preferably between 5 and 1. The local reduction of ∈r is advantageous because for (for example) a radar sensor, in particular in the range of 80 GHz, the antenna elements are to be applied to a region having lower dielectric constant. However, the distribution network is to be situated in an area having higher dielectric constant to minimize emission effects.
As experiments confirm, metallization may be applied to a porous surface of this type via conventional thick-layer technology. However, the implementation of metallized structures in thin-layer or thin-film technology is problematic, these structures having a typical thickness in the range from 500 nm to 3 μm and tending to require a lateral structural precision in the micrometer range for ultra-high-frequency applications. In particular, it has proven problematic in the implementation of thin-film structures applied to the pore and/or tube structure that the cavities of the cavity structure have lateral dimensions up into a magnitude of multiple micrometers. Large-volume cavity structures, i.e., structures in which each individual cavity has a comparatively large extension, arise in particular in the event of long etching times, which are necessary to generate a large porosification depth and a high degree of porosity. These surface structures cause irregularities in thin-film is printed conductors applied thereto, leading to increased specific resistances. The irregularities may also result in the complete breakdown of the function of the thin-film structure in the worst case.
Furthermore, the problem exists that if wet-chemical etching methods are used for the production of the thin-film printed conductors, in certain circumstances the printed conductors are corroded from below. The etching medium penetrates into the tube structure/pores, in particular if the pores and/or tubes of the cavity structure exceed a specific size. The corrosion of the thin-film structure may cause an undesired detachment of the entire metallization layer.